1. Field of the Invention
The present invention relates to a magnetic recording medium having a magnetic thin film as a magnetic layer. More particularly, it relates to a perpendicular magnetic recording medium having excellent running durability even under conditions at a high humidity.
2. Background of the Invention
There has recently been proposed a perpendicular magnetic recording system using a magnetic recording medium having an axis of easy magnetization perpendicular to the surface of the magnetic layer. This perpendicular magnetic recording system is said to be for the most part quite suitable for high density recording since, in this system, the higher the recording density becomes, the greater the reduction in the demagnetization field of a magnetic recording medium, resulting in an excellent reproduced output.
A magnetic recording medium of a thin metal film type having an axis of easy magnetization perpendicular to the surface of a magnetic layer thereof is needed to perform perpendicular magnetic recording. In this connection, there is known a magnetic recording medium of a thin metal film type comprising a support composed of non-magnetic materials such as a high molecular weight material (polymer) or a non-magnetic metal overlaid with a layer of a Co-Cr alloy deposited by a vapor deposition method, a sputtering method or an ion plating method and the like (hereinafter, these methods are referred to as "vacuum thin film forming method").
Also there is known a perpendicular magnetic recording medium of so called two layered type. In this recording medium, a layer of high magnetic permeability composed of soft magnetic materials such as, for example, a permalloy layer (Ni-Fe type alloy) underlies the perpendicular magnetic layer composed of Co-Cr alloy as described above. This structure improves the recording and reproducing efficiency during perpendicular magnetic recording and reproducing.
However, such a magnetic recording medium of a thin metal film type has serious problems of poor running durability and poor wear resistance. To solve the above problems, it has been proposed to provide a protective layer made of a thermoplastic resin or a thermosetting resin on a surface of a magnetic layer. However, the above proposal is not satisfactory because the thickness of the protective layer is limited due to a spacing loss between a head and a magnetic layer.
It is proposed in Japanese Patent Publication No. 33521/1979 that a carbon protective layer be provided by a vapor deposition method to improve wear resistance of a magnetic plated layer of a magnetic recording medium.
When such a carbon protective layer is provided on the magnetic recording medium of a thin metal film prepared by a vacuum thin film forming method, running durability under a normal humidity condition is improved. However, running durability is greatly degraded under long-term storage or under running conditions at a high humidity. That is, the bonding of a protective layer to a magnetic layer is reduced, for example, under a condition of 80% relative humidity, and the protective layer is easily peeled off or comes off in an extreme case. In this case, running is obliged to stop.